Deprecate WebMouseEvent.windowX and windowY, document other coordinates.
Deprecate WebMouseEvent.windowX and windowY, document other coordinates.

We have 3 sets of coordinates in blink::WebMouseEvent: (x,y),
(windowX, windowY) and (globalX, globalY). The difference between them
is not documented, and we see no reason to have anything more than
window-relative and screen-relative co-ordinates. Among them,
(windowX,windowY) seems redundant: it has ~30 references, and all of
them except 4 can be replaced by (x,y). We are in the process of fixing
the exceptions.

This CL marks (windowX, windowY) as redundant to discourage new usage,
and documents the two other coordinates.

BUG=507787

Review URL: https://codereview.chromium.org/1250863002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@199329 bbb929c8-8fbe-4397-9dbb-9b2b20218538

Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk